The invention relates to an implantable dosing device for the continuous release of medication in the human or animal body, comprising a medicine reservoir of variable volume and a propellant chamber of variable volume tensionally connected to the medicine reservoir, as well as flow control means.
In a number of diseases it is necessary to dispense medication to the patient over extended periods of time. The dispensing of Insulin in the case of diabetes, corticosteroids in the case of rheumatic diseases or cytostatica in the case of cancer are examples of such medications. These medications have heretofore been delivered to the patient's body predominantly orally or by injection at certain time intervals. Such delivery of medication is, therefore, intermittent and is only imperfectly matched to the actual requirement of the patient. With many medications, it is in addition very important that the dosing is very accurate, as an excess, as well as deficiency, can have detrimental effects.
Accordingly, a number of devices for delivering medication to the human or animal organism have already been developed, the purpose of which is to allow better dosing of medication. Of these devices, mechanical pumping systems have the disadvantage that they have movable parts such as gears and valves which are subject to wear in operation and in most cases do not meet the requirements which are desired as to dosing accuracy, service life and tightness.
An implantable dosing device, a so-called infusion pump, is known, which has in a housing two chambers which are separated from each other by a liquid- and vapor-tight intermediate phase forming a pressure connection (German Offenlegungsschrift No. 2,124,062 or U.S. Pat. No. 3,731,681). The first of these two chambers, which has an inlet opening which is closed-off liquid- and vapor-tight, is partially filled with a stable, volatile liquid, which exerts a vapor pressure which is higher than one atmosphere at physiological temperatures. Such liquids are, for example, perfluoropentane, tetramethyl silane, ethyl ether and methylformiate. The second chamber has an inlet opening which is closed-off liquid-tight, and at least one release opening which is in direct fluidic connection with a flow-regulating resistance element. The flow control, which is also in direct fluidic connection with a line which leads to at least one infusion location, is preferably a capillary tube; however a porous plug or a filter may also be used.
In this known dosing device, the medium to be infused is transported by the vapor pressure of a liquid. As this vapor pressure is constant for a given (body) temperature, a substantially constant flow velocity will in general adjust itself which, while dependent on the type of flow regulator used, is also predetermined by its nature. Thus, the delivery of the medicine to the body cannot be controlled or regulated which is disadvantageous since regulation is typically necessary or desirable. For example, in the case of diabetics, the Insulin requirement is usually greater during the day than during the night. In addition, the known infusion pump has the disadvantage that the pumped volume depends on the body temperature, i.e., the pumping is not uniform if there are temperature variations. In the event of fever, there is also the danger of overdosing and thus, possibly, danger to the patient.